The Progressive add-a-pack replaces the lower overload springs on the factory pack. This is the 1″ thick leaf that doesn’t follow the contour of the rest of the pack. In removing this spring “wall” and replacing it with a mini progressive leaf pack, the capacity of the truck remains unchanged, as it’s determined by the factory springs, while the progression of the rear spring rate becomes substantially more predictable. The engagement and disengagement of the spring becomes quite a bit smoother increasing both ride quality and traction. As the “wall” was removed, the rear may sit lower with a heavy load BUT it will ride better loaded.
That said, the rear of the Super Duty can be outfitted with a replacement Full Progressive Spring Pack (reduces capacity to a “light” ¾-ton) or this factory leaf-pack modifying Progressive Add-a-Pack. This is where the F250 and F350 are a bit different. Since 2008, the main spring packs on these two trucks is different; the F250 has a lighter rate spring pack than its 350 counterpart. The F250 Add-a-Pack equipped systems are a nice improvement over stock; the F350 with the Add-a-Pack is still very stiff (in some cases, more so than stock) in the rear as the main pack doesn’t move much unless loaded. For this reason, we DO NOT recommend these be installed on an F350.
For optimal ride quality, we recommend the Full Replacement Leaf Spring packs for both trucks but especially the F350 given the super-stiff factory springs. Note that this will require high-end shocks tuned for the lighter rate leaf springs (Backcountry or Pintop). If capacity is a concern, an aftermarket airbag system should be installed to compliment the Full Progressive Leaf Springs to restore the F350 capacity. In the 4.5″/5.5″ Range, we offer our Long Travel Airbags. If 2.5″/3.5″ height, we recommend a system that will allow the airbags to disconnect from the lower mount during suspension cycling to ensure you’re able to take advantage of all your rear suspension travel.